Split toolkit tests into their toolkits

Conceptually, every subdirectory of `mpl_toolkits` is an independent
thing (mostly, though `axisartist` does use parts of `axes_grid1`). It
is thus strange for all the tests to be stuck together in a single
`tests` directory.

Additionally, this causes naming oddities such as using parts of file
names as a pseudo-namespace (i.e., `test_axisartist_*`) when we already
have a namespace, namely the directory that is the toolkit.

As we add additional tests for the toolkits (mostly 3D, I mean), this
will either cause the number of these oddities to grow, or else
`test_mplot3d.py` will just become an extremely large file.

Author: QuLogic